1. Field of the Invention
The present invention relates to a surface acoustic wave element, a surface acoustic wave device, a duplexer, and a method of making a surface acoustic wave element.
2. Related Background of the Invention
Piezoelectric substrates employed in surface acoustic wave devices (hereinafter referred to as “SAW device”) have been known to have pyroelectric property, thereby generating an uneven electric charge distribution on their surface when heated to 80° C. or higher during a heating process or during their use.
When such an uneven electric charge distribution is generated, a potential difference is caused between a plurality of interdigital transducer electrodes (hereinafter referred to as “IDT electrode”) formed on the substrate, whereby discharging for alleviating the potential difference occurs. The discharging may melt the IDT electrodes, thereby causing characteristics to deteriorate, such that the frequency characteristic of the device shifts from its desired characteristic, or short-circuiting occurs. For example, there are cases where the melting of IDT electrodes is seen in 30% or more of SAW devices in a visual inspection after their making. In particular, such melting of IDT electrodes by discharging is likely to occur in recent SAW devices in which IDT electrodes are formed densely in order to respond to higher frequencies.
Therefore, SAW devices having ameliorated the uneven electric charge distribution in the piezoelectric substrate have been disclosed in Japanese Patent Application Laid-Open Nos. HEI 10-107573, HEI 10-163802, HEI 10-126207, and 2001-168676.
Namely, Japanese Patent Application Laid-Open No. HEI 10-107573 discloses a SAW device in which an undercoat layer having a given value of resistance is formed between a substrate and an IDT electrode. Japanese Patent Application Laid-Open No. HEI 10-163802 discloses a SAW device in which a protective layer having a given value of resistance is formed on an IDT electrode. Japanese Patent Application Laid-Open No. HEI 10-126207 discloses a SAW device in which a semiconductor layer having a given value of resistance is formed on an IDT electrode. Japanese Patent Application Laid-Open No. 2001-168676 discloses a SAW device in which a substrate surface located between IDT electrodes is doped.
However, the following problem exists in the conventional SAW devices mentioned above. Namely, it is quite difficult to control the respective thicknesses of the undercoat layer, protective layer, and semiconductor layer formed on the piezoelectric substrate uniformly with a high accuracy. And, it is quite difficult to control the doping depth of the substrate surface likewise. Therefore, in many cases, the respective thicknesses of the undercoat layer, protective layer, and semiconductor layer or the doping depth become uneven, and the undercoat layer and the like fail to function sufficiently for homogenizing the electric charge distribution because of this unevenness. Hence, the conventional SAW devices fail to sufficiently restrain IDT electrodes from melting by discharging.
For overcoming the above-mentioned problem, it is an object of the present invention to provide a surface acoustic wave element, a surface acoustic wave device, a duplexer, and a method of making a surface acoustic wave element which significantly restrain characteristics from deteriorating.